1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices for dynamically controlling the shooting of air guns of a marine source array.
2. Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, which is information especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, commonly used seismic sources are essentially impulsive (e.g., air guns that hold compressed air that is suddenly allowed to expand). An air gun produces a high amount of acoustics energy over a short time. Such a source is towed by a vessel at a certain depth along direction X. The acoustic waves from the air gun propagate in all directions. The air gun instantaneously releases large peak acoustic pressures and energy. Such a source is illustrated in FIG. 1. This figure shows a source array 104 being towed behind a vessel 101. When the source array is activated, acoustic energy is coupled into the water and transmitted into the earth, where part of the energy is partially reflected back from the ocean bottom 113 and from rock formation interfaces 112 (rock layer that has a change in acoustic impedance). Sensors or receivers 106 used to record the reflected energy include hydrophones, geophones and/or accelerometers. The receivers can be encapsulated in either fluid filled or solid streamers 105 that are also towed by vessels at shallow depth.
Returning to the air guns, an air gun stores compressed air and releases it suddenly underwater when fired. The released air forms a bubble (which may be considered spherical), with air pressure inside the bubble initially greatly exceeding the hydrostatic pressure in the surrounding water. The bubble expands, displacing the water and causing a pressure disturbance that travels through the water. As the bubble expands, the pressure decreases, eventually becoming lower than the hydrostatic pressure. When the pressure becomes lower than the hydrostatic pressure, the bubble begins to contract until the pressure inside again becomes larger than the hydrostatic pressure. The process of expansion and contraction may continue through many cycles, thereby generating a pressure (i.e., seismic) wave. The pressure variation generated in the water by a single source (which can be measured using a hydrophone or geophone located near the air gun) as a function of time is called the near-field signature and is illustrated in FIG. 2. A first pressure increase due to the released air is called primary pulse and it is followed by a pressure drop known as a ghost. Between highest primary pressure and lowest ghost pressure there is a peak pressure variation (P-P). The pulses following the primary and the ghost are known as a bubble pulse train. The pressure difference between the second pair of high and low pressures is a bubble pressure variation Pb-Pb. The time T between pulses is the bubble period.
Single air guns are not practical because they do not produce enough energy to penetrate at desired depths under the seafloor, and plural weak oscillations (i.e., the bubble pulse train) following the primary (first) pulse complicates seismic data processing. These problems are overcome by using arrays of air guns, generating a larger amplitude primary pulse and canceling secondary individual pulses by destructive interference.
FIG. 2 represents a situation in which the bubble generated by a single air gun drifts slowly toward the surface, surrounded by water having the hydrostatic pressure constant or slowly varying as the bubble slowly drifts upward. However, when another air gun is fired simultaneously in proximity to the first air gun, the hydrostatic pressure is no longer constant or slowly varying. The bubbles of neighboring guns affect each other.
A source array includes plural individual wave sources. An individual wave source may be an air gun or a cluster of air guns. Since the dimensions of the source array, including plural individual sources, are comparable with the generated wave's wavelength, the overall wave generated by the source array is directional, i.e., the shape of the wave, or its signature varies with the direction until, at a great enough distance, the wave starts having a stable shape. After the shape become stable, the amplitude of the wave decreases inversely proportional to the distance. The region where the signature shape no longer changes significantly with distance is known as the “far-field,” in contrast to the “near-field” region where the shape varies. Knowledge of the wave source's far-field signature is desirable in order to extract information about the geological structure generating the detected wave upon receiving the far-field input wave.
In order to estimate the source array's far-field signature, an equivalent notional signature for each individual source may be calculated for each of the guns using near-field measurements (see e.g., U.S. Pat. No. 4,476,553 incorporated herewith by reference). The equivalent notional signature is a representation of an amplitude due to an individual wave source as a function of time, the source array's far-field signature being a superposition of the notional signatures corresponding to each of the individual sources. In other words, the equivalent notional signature is a tool for representing the contribution of an individual source to the far-field signature, such that the individual source contribution is decoupled from contributions of other individual wave sources in the source array.
However, the stability and reliability of the far-field signature depends on the stability of each of the individual wave sources and of the source array's geometry. During a seismic survey, the individual wave sources' behavior may change (e.g., firing later or earlier than expected, than desirable, or at a smaller amplitude than nominally designed) and thus affect the far-field source signature. In practice, the gun controllers use a sensor called time-break (hereby called TB) installed inside each air-gun body to monitor the launch of each gun. However, for guns of different sizes, different models and/or different service time and maintenance conditions, the delay between the launch (electrical signal sent to gun and valve begins to open) and the actual shot (air goes out of the gun body and begins to generate the shock wave of FIG. 2) may vary.
It would be desirable to have methods and apparatuses capable of controlling and adjusting the firing of the individual wave sources of a marine source array so that all the individual wave sources are fired at the same time, thus, resulting in an improved far-field signature.